Weird Corners... What happened to my Sky90?

Sky90 with F4.5 reducer/flattener, 1min x 10, unguided, 5DII modified

Full Image:

Centre crop

 Top right, bottom right, top left, bottom left

To demonstrate the problem this is an image taken in 2008, same scope and reducer, Mamiya 645 E200, manually guided 1 hour.